The invention relates to a method according to the precharacterizing clause of patent claim 1. Such a method is disclosed in EP-A-0 343 739.
For compatible transmission of supplementary data for a higher level within the normal television text cycle, it has been disclosed (DE-C-39 14 697) for the supplementary data to be transmitted in so-called supplementary pages. According to a proposal (DE-A-42 14 750) which has not been published, specific error protection information, which allows a plurality of errors to be corrected, is transmitted together with the supplementary data, for correction of transmission errors.
For compatibility reasons, the supplementary pages have the same address frame, which is protected using the Hamming code (8, 4), as the pages for the normal "Level 1" pages. Such a protected address frame for "Level 1" pages is disclosed in EP-A-0 343 739. The Hamming code is able to identify and to correct a one-bit error within one byte of the address frame; on the other hand, a double error within one byte can only be identified, but cannot be corrected. Thus, for example, in the case of such a double error in the line address, all the associated supplementary data can no longer be assigned to the relevant line by the decoder, so that they are lost. In this error case, it is necessary to wait until the relevant supplementary page is received again. If, on the other hand, a double error occurs only in one byte of the supplementary data itself, then this double error can be corrected completely, and without any waiting time, by the additionally transmitted error protection information. In consequence, the decoding of the address frame which is provided in the teletext specification has been found to be a bottleneck which partially destroys the error protection measures, which are provided in the abovementioned document for the supplementary data.
It is not feasible to overcome the bottleneck by modifying the address frame, for compatibility reasons. In addition, improved error protection of the address frame necessitates correspondingly increased redundancy which reduces the available useful data rate and increases the waiting time for a desired teletext page correspondingly.
In contrast, the object of the invention, in the case of a method of the type mentioned initially, is to achieve greater decoding reliability for the address frame of a teletext page, in particular a page having supplementary data for a higher level, without any additional error protection measures.
This object is achieved according to the invention by the features specified in patent claim 1.
Advantageous refinements and developments of the method according to the invention result from the subclaims.
In the case of the method according to the invention, the "address frame" bottleneck is improved in that the disturbed data, which have just been received, of the address frame are compared with one another [sic] with the previously received data which have been predicted in a special estimation and are possibly replaced by the predicted data. This makes it possible for a teletext decoder to reconstruct the address frame without errors even in the case of disturbances which go beyond the error protection which is additionally transmitted.